By that I mean can we use QM equations for calculating classical objects moving assuming their sizes >> ħ? Like we can use relativistic equations for calculation of the train speed + speed of a person in it if we want.

trying to see how the very small, quantum mechanics, and the very large, general relativity or newton's 2nd law, work or relate together, referred to as the theory of everything, has never been solved.

It is my impression (from bits and pieces I have gleaned here and there) that when the QM equations are applied to macroscopic objects, the terms that produce the quintessential QM effects become negligible.

For example (I have heard that) if one were to calculate the Quantum Wave-Equation for a car driving down a road, the Wavelength would be measured in Light-Years and the amplitude would be so tiny as to approach the Planck limit.

Thus I believe that Newtonian and QM can be reconciled by recognising that certain QM terms become negligible at larger scales.

Similarly I believe that it is possible to reconcile GR and Newtonian, when the local gravitational fields and relative speeds are small.

Problems occur, however when one tries to complete the link between GR and QM.